Method for producing a grid structure, an optical element, an evanescence field sensor plate, microtitre plate and an optical communication engineering coupler as well as a device for monitoring a wavelength

ABSTRACT

For producing a coupling grating formed as a line grating with a grating period between 100 nm and 2500 nm, a substrate ( 1 ) is covered with a photoresist layer ( 10 ) and exposed for instance at the Lithrow angle (Θ L ) or at 0° to a mercury-vapour lamp ( 11 ) via a folding mirror ( 13, 13 ′) through a phase mask ( 14 ) in the near field of which the photoresist layer is arranged, then structured by reactive ion etching and provided with a transparent layer by reactive DC magnetron sputtering, particularly pulsed DC sputtering or AC-superimposed DC sputtering. The phase mask ( 14 ) is structured in advance with the laser two-beam interference method. Since highly precise gratings can be produced even in large dimensions, the process is particularly suited for the production of optical elements, particularly evanescent field sensor plates and optical couplers for communications technology which can be employed in particular as filters for wavelength multiplexing in fibre-optic networks.

FIELD OF THE INVENTION

[0001] The invention relates to a process for producing at least onecontinuous grating structure according to the preamble of claim 1, anoptical element produced with the aid of the process according to theinvention, as well as an evanescent field sensor plate and a microtitreplate and an optical coupler for communications technology, furthermorea device for monitoring a wavelength containing such a coupler.

[0002] The process according to the invention is suitable for producingoptical elements with grating structures. Evanescent field sensor platesand microtitre plates produced according to the process can be used inchemical and biomolecular analyses. The applications of optical couplersare in communications technology, and more particularly in datatransmission via fibre networks. A potential application of a particularcoupler according to the invention is in a device for monitoring thewavelength of laser light in a fibre network.

PRIOR ART

[0003] From EP-A-0 602 829 a process for producing a grating structureon a substrate, for example for a DBR semiconductor laser, is known inwhich first a phase mask is produced and then the substrate, e.g. InP,is exposed at the Lithrow angle through the phase mask. The exposure canbe to a Hg—Xe arc lamp having a light source diameter of 0.25 mm, threelines around 365 nm wavelength being filtered out. The substrate islocated in the near field of the phase mask, i.e. at a distance of atmost 10 μm.

[0004] To produce the phase mask, a quartz substrate is covered withthree layers, a photoresist layer, a thin germanium layer and finally alayer of a resist sensitive to electron beams. The uppermost layer isthen structured by electron beam writing, developing the uppermost layerand removing the unexposed parts. The structure is transferred to thelayers underneath by reactive ion etching, initially with CF₃Br and thenwith O₂, and finally to the quartz substrate itself by a further step ofreactive ion etching, whereupon the residues of the layers are removed.The grating period may be, for example, between 190 nm and 250 nm. Thephase mask may be several centimetres long and the grating may extendover its entire length. However, as a rule, the length of the lines isonly 5-20 μm. Greater lengths are possible but require very longprocessing times. In practice, gratings of more than 1 mm² can scarcelybe produced with reasonable effort and good accuracy. In particular,stitching errors can hardly be avoided during electron beam writing.

[0005] From U.S. Pat. No. 5,675,691 a plate is known on which couplinggratings are produced by applying a layer of TiO₂, Ta₂O₅, HfO₂, Y₂O₃,Al₂O₃, Nb₂O₅, nitride or oxynitride of Al, Si or Hf to a substrate ofglass, in particular quartz glass, ceramic, or predominantly organicmaterial, it being possible to provide a 20 nm thick intermediate layer,e.g. of SiO₂, and to structure it by ablation or modification of therefractive index by means of exposure to two superimposed beams of anexcimer laser or to a beam modified by a mask. Instead, it is alsopossible to structure an intermediate layer, e.g. of TiO₂, in which theablation barrier is lower and which is applied either to the layer ordirectly to the substrate and, in the latter case, is superimposed bythe layer after structuring. The grating periods are, for example, 375nm or 440 nm. The grating surface area is freely selectable and may be,for example, 1 mm×1 mm or 8 mm×8 mm.

[0006] From U.S. Pat. No. 5,822,472 an evanescent field sensor plate forchemical analyses is known which bears a 40 nm to 160 nm thick layer ofTiO₂, ZnO, Nb₂O₅, Ta₂O₅, HfO₂ or ZrO₂ on a support of plastic, glass, orquartz. An intermediate layer of nonluminescent material with a lowrefractive index, e.g., quartz having a thickness of 100 nm, forexample, which at the same time serves as an adhesion promoter, may bearranged in between. An input coupling grating and an output couplinggrating are provided which are created by known photolithographic orholographic and etching methods, either in the support or in the layer,and have a grating period of between 200 nm and 1000 nm. The gratingsmay have dimensions of 2 mm (parallel to the lines)×4 mm, with a totalsurface area of the wave-guide plate of 12 mm×20 mm.

[0007] From J. Dübendorfer and R. E. Kunz: “Compact integrated opticalimmunosensor using replicated chirped coupling grating sensor chips”,Applied Optics, 37/10 (Apr. 1, 1998), a further evanescent field sensorplate comprising a polycarbonate support plate is known into which amodulated input coupling grating having a grating period varying between420 nm and 422.8 nm and an output coupling grating having a gratingperiod varying between 595.1 nm and 600.8 nm were embossed. Thereafter,a TiO₂ layer having a thickness of 137 nm and a refractive index of2.346 was applied by means of low-temperature DC magnetron sputtering,and finally the evanescent field sensor plate was silanised. The inputcoupling angle is about−9.5° and the output coupling angle is about22.5°.

[0008] From U.S. Pat. No. 5,738,825 a microtitre plate can be gatheredwhich has a 20 nm to 1000 nm, preferably 30 nm to 500 nm thick layer ofTiO₂, Ta₂O₅, HfO₂, ZrO₂, SiO₂, Si₃N₄, Al₂O₃, Nb₂O₅, nitride oroxynitride of Al, Si or Hf applied to its bottom surface, this layerbeing covered by a plastic layer. Input and output coupling gratings aremounted underneath each cavity. The gratings have a grating periodbetween 330 nm and 1000 nm, in particular about 400 nm to 800 nm, andare produced by lithographic or mechanical methods.

[0009] From CH-A-688 165 a wave-guide plate comprising a substrate ofplastic, e.g. polycarbonate, is known whose surface was structuredmechanically—by deep drawing, embossing or during its injectionmoulding—and in particular provided with a coupling grating, and bears alayer of TiO₂, Ta₂O₅, ZrO₂, Al₂O₃, SiO₂—TiO₂, HfO₂, Y₂O₃, Nb₂O₅, siliconnitride, oxynitride, SiO_(x)N_(y), HfO_(x)N_(y), AlO_(x)N_(y),TiO_(x)N_(y), MgF₂ or CaF₂ applied by a PVD method. To reduce theattenuation losses, an approximately 20 nm thick intermediate layerapplied to the substrate prior to the layer and comprising an inorganicdielectric material such as SiO₂ is provided which at the same timeserves as an adhesion promoter.

[0010] All plates described above are produced by processes with whichno satisfactory uniformity of the coupling grating can be achieved, sothat the coupling angle varies relatively widely. Consequently, therelative angular position of the exposure unit and plate must beoptimised laboriously in each step when the plate is to be used as anevanescent field sensor plate. The filter characteristics areunsatisfactory and not sufficient, for instance, for selectivelyfiltering a particular wavelength from a group of very closely spacedwavelengths, when the plate is used as an optical coupler incommunications technology. Some of the processes described are also verylaborious or do not allow very large numbers of pieces of constantquality to be made.

SUMMARY OF THE INVENTION

[0011] It is the object of the invention to provide a process whichpermits the production, particularly the volume production, of latticestructures with high precision and relatively low effort. This object isachieved by the features in the characterizing clause of Claim 1. Usingthe process according to the invention it is also possible to producelarge-area lattice structures, particularly continuous gratingstructures which are long in a direction parallel to the lines, withhigh precision throughout, and in a simple and economical way. Moreover,a large freedom of design is secured with respect to the arrangement andshape of the grating structures. Hence the production of highlydifferent optical elements for diverse applications is feasible with oneand the same installations. The process according to the inventionfurthermore permits the production of large series of optical elementsof constant quality and with optical properties such as couplingefficiencies and, in particular, coupling angles which are constantwithin narrow limits.

[0012] Furthermore, a highly precise optical element is to be providedsuch as can be produced by the process according to the invention. Inparticular, the optical element can be formed as an evanescent fieldsensor plate or as a microtitre plate based on such a plate. In view ofthe narrow limits within which, even in long gratings, the couplingangle varies, it is possible to simultaneously illuminate and read outlarger parts of the evanescent field sensor plate or microtitre plate.Successive exposure of different parts of the evanescent field sensorplate or microtitre plate is also simplified since reoptimisation of therelative angular position of this plate and the exposing unit is notrequired or in any case greatly simplified.

[0013] The optical element can also be formed as an optical coupler forcommunications technology. In this case, the high precision present evenin large grating structures guarantees excellent filter characteristics,and particularly a very-narrow-band selection of individual wavelengths,so that for instance a wavelength multiplexing involving very closelyspaced wavelengths is possible, which raises the transmission capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention is explained in more detail below with reference tofigures merely representing an embodiment.

[0015]FIG. 1 shows a plan view of an evanescent field sensor plateaccording to the invention, having an added top piece indicated bydashed lines so that it becomes a complete microtitre plate,

[0016]FIG. 2 shows a section along II-II in FIG. 1,

[0017]FIG. 3 schematically shows the use of a microtitre platecomprising an evanescent field sensor plate according to the invention,

[0018]FIGS. 4a-e show different steps in the production of an evanescentfield sensor plate according to the invention,

[0019]FIG. 5 schematically shows the setup used for exposing thephotoresist layer during production of the evanescent field sensor plateaccording to the invention,

[0020]FIG. 6 shows the photo mask and substrate with the photoresistlayer under exposure, and

[0021]FIGS. 7a-g show different steps in the production of a phase maskfor the production of an evanescent field sensor plate according to theinvention.

[0022]FIG. 8a schematically shows a plan view of a first embodiment ofan optical coupler for communications technology according to theinvention,

[0023]FIG. 8b schematically shows a central section through the coupleraccording to FIG. 8a,

[0024]FIG. 9a schematically shows the reflectivity of the coupleraccording to the first embodiment,

[0025]FIGS. 9b-d show diagrams showing the selection of a wave-length bythe coupler according to the first embodiment,

[0026]FIG. 10 shows an arrangement with a coupler according to the firstembodiment,

[0027]FIG. 11a schematically shows a plan view of a second embodiment ofan optical coupler for communications technology according to theinvention,

[0028]FIG. 11b schematically shows a central section through the coupleraccording to FIG. 11a,

[0029]FIG. 12 shows an arrangement with a coupler according to thesecond embodiment,

[0030]FIG. 13a schematically shows a plan view of a third embodiment ofan optical coupler for communications technology according to theinvention,

[0031]FIG. 13b schematically shows a central section through the coupleraccording to FIG. 13a,

[0032]FIG. 14 shows a device for monitoring and stabilising thewavelength of a light beam emitted by a laser which comprises a coupleraccording to the third embodiment, and

[0033]FIG. 15 shows the transmissivity of the coupler according to thethird embodiment as a function of its position on the coupling gratingat different wavelengths.

DESCRIPTION OF EMBODIMENTS

[0034] The process according to the invention will in the following beexplained in more detail in connection with an evanescent field sensorplate and its production. Here, an evanescent field sensor plate isunderstood as a plate making it possible by illumination of one part ofthe surface to create an evanescent field in the reach of which a sampleto be analysed may be arranged. Therefore, evanescent field sensorplates serve the purposes of chemical analysis. Evanescent fields arenonradiating electromagnetic fields which fall off and vanish withincreasing distance from the scattering surface at which they aregenerated. Such fields can arise in connection with spatial modulationsof the electric field in the plane having periodicities smaller than onewavelength. The best-known example of such a modulation occurs at aninterface between a dielectric and air when a light beam coming from theside of the dielectric has an angle of incidence larger than thecritical angle.

[0035] Evanescent field sensor plates have a transparent light-directinglayer from which the evanescent light leaves their surface, andinteracts with bound molecules present there, for instance excitingtheir luminescence. The evanescent field sensor plate consists (FIGS. 1,2, the diagrams are schematic and not to scale) of a glass substrate 1,for example AF 45 of Schott DESAG measuring 102 mm×72 mm and having athickness of 0.7 mm, which on one side bears a transparent layer 2 ofTa₂O₅ having a thickness of 150 nm. Its refractive index is 2.11 at awavelength of 633 nm.

[0036] A plurality of coupling gratings 3 in the form of parallel,spaced apart strips which extend parallel to the lines over the entirewidth of the evanescent field sensor plate are arranged on the surfacebearing the layer 2. The width of each of the strips is 0.5 mm. Thegrating period Λ=360 nm, the groove-to-land ratio is about 1:1, and thegrating depth is about 20 nm. The parameters defining the grating arevery precisely preserved over the full length of all strips.Consequently, changes in the coupling angle Θ at which a light beamdirected from below through the substrate 1 to the coupling grating 3,in particular a light beam having a wavelength of about 633 nm, iscoupled into the layer 2 with maximum coupling efficiency remain withinvery narrow limits. Along the lines of a coupling grating 3, it changesby at most 0.05°/cm. Over the entire evanescent field sensor plate, thedeviation of the coupling angle Θ from a mean value which, in the casedescribed, corresponds to 2.31° remains below 0.15°.

[0037] The surface of layer 2 is provided with a coating consisting ofan adhesion promoter layer, preferably of at least one chemical compoundselected from the group of silanes, epoxides, and self-organisingfunctionalised monolayers, and on top of it a layer of molecularrecognition elements such as nucleic acids, antibodies, antigens,membrane receptors, and their ligands.

[0038] As shown in FIG. 3 and indicated in FIG. 1, the evanescent fieldsensor plate is extended by an added honeycomb-like top piece 4 ofplastic to form a microtitre plate to be employed for chemical analysis,in particular of biological substances. The top piece has a cover plate5 which is perforated by round openings 6 having a diameter of, forexample, about 8 mm which are arranged as a regular array. At the bottomside of cover plate 5, a tube section 7 is attached to each of theopenings which is open at the bottom, laterally delimits a cavity 8, andis tightly bonded, for example glued, at its lower end to the evanescentfield sensor plate 2.

[0039] If it is intended to investigate the contents of a cavity 8, forinstance an analyte such as blood, serum, urine, saliva, or a solutioncontaining a drug candidate, with respect to the concentration ofspecific molecules, an adjacent coupling grating 3 of the evanescentfield sensor plate 2 is exposed in a manner known per se to a suitablelight source at the coupling angle Θ, the light having a specificwavelength, in the example to light with a wavelength of 633 nm, bymeans of a He—Ne laser. The light is conducted through the layer 2forming the bottom of the cavity 8 to the adjacent coupling grating 3′,and there coupled back out. The evanescent light stimulates fluorescencein molecules in the cavity 8 which are bound to recognition elements,which fluorescence is recorded by an optical system 9 and then analysed.The high precision with which the coupling angle Θ is preserved over thelength of the coupling grating 3 permits a simultaneous and highlyefficient examination of the cavities arranged along the same. Sinceover the entire evanescent field sensor plate 2, the coupling angle Θdeparts but slightly from the mean value, no laborious optimisation ofthis angle is required even when examining the next row of cavities 8.As an alternative or in addition to evaluating the fluorescence, one canalso record changes in the refractive index produced at the bottom ofcavity 8 by the binding of molecules to the recognition elements.

[0040] As shown schematically in FIGS. 4a-e, to produce the transparentlayer 2, one first applies a photoresist, e.g. AZ1518, diluted 1:4 withAZ1500, both of Clariant, at 1300 rpm to the substrate 1 and then bakesfor 30 min at 90° C. in an oven, whereupon Aquatar of the samemanufacturer is applied at 1400 rpm and once again baked for 15 min at90° C. in the oven. The photoresist layer 10 thus produced has athickness of less than 200 nm, as a rule of about 150 nm, so thatinterfering standing waves will not develop in it. The reflectivity isbelow 1%, so that interfering reflections which could lead to thedevelopment of Newton's rings are also practically excluded (FIG. 4a).

[0041] In a subsequent step, the photoresist layer 10 is exposed for 70s. For this purpose, the substrate 1 is introduced into the setupaccording to FIG. 5, which is a mask aligner MA4 of Süss, Munich,comprising a modified mercury-vapour lamp 11 having a modified end-stageoptical system 12 and folding mirror 13. The optical system 12 comprisesa bandpass filter which, for example, filters out the I-line at awavelength of 365 nm, and a polarizer, which preferably producess-polarised light. To improve the parallelism of the beams, the fly'seye is removed from the beam path, a lamp with as small an arc aspossible is used and is positioned as far away from the substrate aspossible.

[0042] The exposure occurs through a phase mask 14. It comprises asubstrate of transparent material, in the example quartz, with alarge-area grating structure, a diffraction grating bearing a layer ofnontransparent material, in the example chromium, which isinterrupted—in the example, by regularly spaced, consecutive strips.Phase masks of this type are supplied by Ibsen in Farum (Denmark), andare manufactured approximately as follows:

[0043] A quartz substrate 15 is covered with a photoresist layer 16(FIG. 7a) and the latter is exposed to light using the laser two-beaminterference method, and developed (FIG. 7b). Then a full-areadiffraction grating is produced on the surface of the quartz substrate15 by etching and subsequent removal of the photoresist (FIG. 7c). Saidsurface is then completely covered with a chromium layer 17 (FIG. 7d). Acontinuous photoresist layer 18 is then applied to the chromium layer 17(FIG. 7e) and exposed through a mask structured by electron beam orlaser writing. The photoresist is then developed (FIG. 7f) and thechromium layer 17 is removed by etching from the parts not covered byphotoresist. Finally, the residues of the photoresist layer 18 areremoved to finish the phase mask 14 (FIG. 7g). The structure of the maskthus determines which parts of the phase mask are transparent.

[0044] The substrate 1 is arranged underneath the phase mask 14 in sucha way that the photoresist layer 10 is in vacuum contact with said phasemask. The upper side of the phase mask 14 is exposed at an angle whichapproximately corresponds to the Lithrow angle Θ_(L), which is the angleat which the angle of incidence is equal to the angle of first-orderdiffraction, and in particular deviates by no more than 10°, preferablyby no more than 5°, from said angle. Under these conditions, apronounced diffraction pattern whose structure corresponds to that ofthe grating of the phase mask 14 forms in the near field below thetransparent regions of the phase mask 14 (FIG. 6). Alternatively, thephase mask 14 can also be exposed at an angle which approximatelycorresponds to 0°, i.e., vertical incidence, or at an angle that doesnot deviate from it by more than 10°, preferably not by more than 5°(folding mirror 13′ shown in dashed lines). In this case, thediffraction pattern in the near field of the phase mask 14 has half thegrating period of that mask.

[0045] After exposure the Aquatar layer is removed by washing withdeionized water, and the photoresist is then developed (FIG. 4b). Thoseparts of the surface of substrate 1 which are not covered withphotoresist are then etched with Ar and CHClF₂ at a pressure of 0.02mbar in a parallel-plate reactor with capacitive excitation of theplasma at 13.6 MHz and an RF power of 50 W. The etch depth is 20 nm. Thephotoresist is then removed. For this purpose, it is first subjected toreactive ion etching for 60 s in an oxygen plasma at a pressure of 0.2mbar and an RF power of 50 W, then detached with Remover AZ100, Deconex,and deionized water (FIG. 4d).

[0046] Finally the layer 2 is applied by reactive pulsed DC magnetronsputtering or by DC magnetron sputtering superimposed with an ACfrequency between 1 kHz and 1 MHz, in a Balzers MSP1000 unit, similarlyas described in EP-A-0 508 359 (FIG. 4e). This step is carried out in anAr—O₂ atmosphere at a pressure of 3.7 μbar. The target material istantalum. Finally, the evanescent field sensor plate is cut to its finalsize by wafer sawing.

[0047] Particularly on account of exposure through a phase mask that canbe reused practically as often as desired, the process described permitsthe production of elements with grating structures, particularlydiffractive coupling gratings, in large numbers and in a simple manner.The fact that the phase mask is structured by the two-beam interferencemethod also implies that large defect-free grating structures havingsurface areas of 10 cm² and more can be produced on it with highprecision, whereas other structuring methods such as electron beamwriting are not suitable for this purpose owing to their virtuallyunavoidable stitching errors. Therefore, large optical elements withlarge-area gratings of high quality and uniformity can be produced, notonly as final products but also as semi-finished plates which by wafersawing, scribing and breaking or laser cutting can then be separatedinto smaller final products which thus can be produced very economicallyand in high quality.

[0048] Evanescent field sensor plates can of course also be produced ingeometries and optical properties corresponding to other standards orrequirements. Thus, another evanescent field sensor plate can havedimensions of 57 mm×14 mm×0.7 mm and be provided with two strip-shapedcoupling gratings having a width of 0.5 mm each which are symmetricallyarranged in parallel with the long sides, and have a mutual distance of9 mm. The grating period Λ=318 nm, the grating depth 12 nm, whileotherwise the properties of the layer and coupling gratings are the sameas in the first example. In this case the coupling angle Θ=−12.14° at awavelength of 633 nm, varying parallel to the lines by at most 0.15°/cm.The deviation from a mean value remains below 0.5° everywhere on theevanescent field sensor plate. For the production of a semifinishedplate from which the individual evanescent field sensor plates are thenobtained by wafer sawing, a phase mask is employed which measures 150mm×150 mm and has a region with a grating of grating period 318 nmmeasuring 115 mm×115 mm. The regions corresponding to the couplinggratings are bare while the remaining portion of the grating is againmasked by a nontransparent layer, particularly a chromium layer.Otherwise the production proceeds as described above.

[0049] A further example is an evanescent field sensor plate measuring75 mm×113.5 mm×0.7 mm which as to its basic features essentiallycorresponds to FIG. 1, and which bears 13 strip-shaped coupling gratingseach 0.5 mm wide which are parallel to the broadside and have distancesbetween neighboring strips of 8.5 mm each. Layer and grating propertiescorrespond to those of the second example. The coupling angle Θ=−11.48°at a wavelength of 633 nm, varying parallel to the lines by at most0.05°/cm. Over the entire evanescent field sensor plate it departs froma mean value by at most 0.4°. The evanescent field sensor plate can beexpanded to a microtitre plate with 8×12 cavities by adding a suitabletop portion.

[0050] Apart from the embodiments of optical elements having gratingstructures produced by the process according to the invention and usedin chemical analysis, as portrayed above, embodiments for applicationsin communications technology are particularly pertinent. Such elementsare suited above all as highly efficient optical couplers such as thoseemployed in fibre-optic networks.

[0051] A first example of such a coupler is represented in FIGS. 8a and8 b. On a substrate 1 consisting of a glass plate 19 and a layer 20 oftransparent material, a coupling grating 3 formed as a line grating ofconstant grating period is arranged. The layer 20 is covered by atransparent layer 2, consisting for instance of Ta₂O₅. Layer 2 acts as awave-guide. The coupling grating 3 reflects light of a particularwavelength λ_(B) according to the wavelength-dependent reflectivity Rschematically represented in FIG. 9a, while incident light of all otherwavelengths is transmitted. This is shown in FIGS. 9b-d, where FIG. 9ashows the incident wavelengths, FIG. 9b the reflected wavelength λ_(B),and FIG. 9c the transmitted wavelengths. Using the coupler it is thuspossible to filter out a particular wavelength, for instance in afibre-optic network using wavelength multiplexing. Thanks to the highgrating quality, the full width at half maximum of the reflectivity R asa function of wavelength is very small. Hence even with wavelengths veryclosely spaced, it is possible to highly efficiently separate awavelength.

[0052] An example for the use of such a coupler is shown in FIG. 10. Twoparallel stripe waveguides 21 a, b of known structure are runningparallel at a very small mutual distance in a coupling region 22, insuch a way that 50% of the light conducted in the first stripe waveguide21 a is transferred to the second stripe waveguide 21 b, and vice versa.In the final segment of the first stripe waveguide 21 a, a coupler 23with a coupling grating 3 according to FIGS. 8a and b is incorporatedwhich selectively reflects light having a wavelength λ₃.

[0053] When signals having wavelengths λ₁, λ₂, λ₃, λ₄ etc. are fed intothe first stripe waveguide 21 a at an input 24, then on one hand 50% ofall signals in the coupling region 22 are transferred to the secondstripe waveguide 21 b where they are conducted to a first output 25 a,while the signals remaining in the first stripe waveguide 21 a areconducted to a second output 25 b, except for the signal of wavelengthλ₃ corresponding to the λ_(B) according to FIGS. 9a-d which is reflectedat the coupler 23 so that in the coupling region 22, 50% of itsintensity transfer into the second stripe waveguide 21 b where thesignal is conducted in a direction opposite to that of the signalstransferred directly from the first stripe waveguide 21 a, and reaches athird output 25 c where finally it has been isolated and can be furtherprocessed. The signals of outputs 25 a and b can be recombined to asignal differing from the original one only by a 50% attenuation of thesignal with wavelength λ₃.

[0054] The coupler 23 can be completely integrated into the first stripewaveguide 21 a, in such a way that this has the same structure ascoupler 23 and this coupler forms a single part with the first stripewaveguide 21 a. The only distinction of coupler 23 is then its bearingthe coupling grating 3.

[0055] A second example of a coupler is represented in FIGS. 11a and b.In a longitudinal direction on top of a rectangular substrate 1 ofglass, for instance Schott DESAG AF 45 with a refractive index of 1.52,two coupling gratings, an input coupling grating 3 a and an outputcoupling grating 3 b, are arranged consecutively at a mutual distance,each extending over the full width of the coupler. The input couplinggrating 3 a has a grating period of Λ₁=981 nm and a grating depth of 6nm, the output coupling grating 3 b has a grating period of Λ₂=1350 nmand a grating depth of 12 nm. The upper side of substrate 1 is coveredby a continuous transparent layer 2 consisting of Ta₂O₅ and having arefractive index of 2.1. Its thickness is 400 nm.

[0056] The coupler can be used as a drop filter monitoring andstabilising the intensity of a light beam, for instance a line of amultimode laser. To this end (FIG. 12), the coupler 23 described aboveis so arranged between the ends of a first optical fibre 26 a and asecond optical fibre 26 b arranged in the continuation of the formerthat the input coupling grating 3 a faces the end of the latter whilethe former faces the bottom side of coupler 23. The light supplied bythe first optical fibre 26 a passes through the part of the coupler 23carrying the input coupling grating 3 a while a fraction of the light ofthe 1550 nm line corresponding to less than 0.01% of its intensity iscoupled into the layer 2 by said grating. At the output coupling grating3 b, light is coupled out at an angle of 30° and reaches anappropriately disposed photodetector 27 the output signal of which is ameasure of intensity of the monitored line. Owing to the high precisionof the input coupling grating 3 a, the input coupling is highlywavelength-sensitive, the full width at half maximum of the intensitydistribution being a mere 0.01 nm, so that a specific monitoring of asingle wavelength is possible even where the wavelengths are closelyspaced, as desired in wavelength multiplexing in the interest of a hightransmission capacity.

[0057] A third example of an optical coupler according to the inventionis represented in FIGS. 13a and b. A rectangular substrate 1 consists ofa glass plate 19 with a refractive index of 1.586 and a transparentlayer 20 of TiO₂ with a refractive index of 2.4 covering the upper faceof this glass plate in a thickness of 285 nm. The upper face bears acoupling grating 3 occupying its full width which has been produced byremoving layer 20 completely in a pattern of lines, and is covered by afurther transparent layer 2 consisting of MgF₂ which is 342 nm thick andhas a refractive index of 1.38. The grating depth thus corresponds tothe thickness of layer 20, and is 285 nm. The grating period Λ(x) varieslinearly in a direction normal to the grating lines, increasing fromΛ₁=970 nm to Λ₂=977 nm.

[0058] When producing the coupler, layers 20 and 2 can be applied asdescribed in connection with the first example of an evanescent fieldsensor plate. Production of the coupling grating 3 after application ofthe layer 20 also occurs as described there. In this operation, a phasemask is used whose grating varies appropriately, hence linearly in thepresent case. Such phase masks can be produced by appropriately bendinga flexible master copy and applying a grating structure using thetwo-beam interference method. The phase mask is derived by replication,that is, by making an impression of the reflattened master copy.

[0059] The coupler can advantageously be used in a device for monitoringand stabilising the wavelength of a laser 28 (FIG. 14) the light ofwhich is fed into a light-conducting fibre 29, for instance a glassfibre, of a fibre network. The device comprises a semi-transmissivemirror 30 arranged in the path of the light beam coming from the laser28, followed by a first optical system 31 to expand, and a secondoptical system 32 to collimate the part of the light beam deflected bythe mirror 30. Following after the optical systems, the coupler 23described above is arranged in the light beam normal to the beamdirection, in such a way that the beam strikes the coupling grating 3.Mounted directly beneath the coupler 23 is a detector system with twophoto-detectors 33 a and b which are arranged so as to be immediatelyadjacent one behind the other and normal to the lines in such a way thatthe part of the light beam transmitted by a first portion of thecoupling grating 3 where the grating period is between Λ₁ and anintermediate value Λ_(i) strikes the first photodetector 33 a while thepart transmitted by the remaining portion of the coupling grating 3where the grating period is between Λ_(i) and Λ₂ strikes the secondphotodetector 33 b. The photodetectors 33 a and b can be displaced sothat Λ_(i) is adjustable.

[0060] The transmissivity of the coupler 23 is a sensitive function ofwavelength and of the grating period Λ. Because of theposition-dependent variation of the grating period Λ(x), therefore, itexhibits a specific wavelength dependence for the incident light whichdepends on its position. This is shown in FIG. 15, where thetransmissivity T can be gathered as a function of position on thecoupling grating 3 for three very close wavelengths (1549.5 nm, 1550 nm,1550.5 nm). The minimum of the transmissivity curve shifts to larger orsmaller grating periods as the wavelengths increase or decrease, andhence to a different position on the grating. This in turn gives rise tochanges in the relative intensities of the light captured by thephoto-detectors 33 a and 33 b, which has a direct effect on the size oftheir output signals I_(a), I_(b).

[0061] For the purposes of stabilising a particular wavelength, one canthen roughly adjust the position of the detector arrangement inaccordance with the wavelength of interest, and then calculate a value

Q=(I _(a) −I _(b))/(I _(a) +I _(b))

[0062] and reduce it to zero by shifting the detector arrangement. Anychange in wavelength of the light beam coming from the laser 28 willgive rise to a positive or negative deviation of the value of Q fromzero, depending on the direction of the wavelength change, and can becompensated by corresponding control of the laser 28. The intensity ofthe light beam is unimportant here. Only the intensity distribution ofthe expanded light beam which may not be homogeneous but follow agaussian distribution, for instance, might eventually causeperturbations, but this can then be compensated by appropriatearrangement or extension of the optical systems or by calculation.

[0063] The optical elements according to the invention can be modifiedin many respects without departing from the basic concept of theinvention. Thus, in many cases deviations from the mean value of up to0.3° or even up to 0.5° over the entire element or even over a couplinggrating can be admitted. For the evanescent field sensor plates, too, itwill often be sufficient when the changes in coupling angle Θ along thegrating lines are not larger than 0.1°/cm.

[0064] Many deviations or special adaptations to particular requirementsare possible as well in the production process. Thus, even in theexposure step which is decisive for the process, the photoresist layermay by spaced apart from the phase mask, which facilitates the process.However, it must be arranged in the near field, that is, at a distancewhich as a rule is smaller than 100 μm, for the diffraction pattern tobe sufficiently pronounced. This distance may perhaps be between 2 μmand 100 μm. Instead of a mercury-vapour lamp, a laser can also be usedas the light source, particularly an excimer laser or an argon laser.Apart from Ta₂O₅, other substances can be used as materials for thelayer, particularly Nb₂O₅, TiO₂, ZrO₂, Al₂O₃, SiO₂—TiO₂, HfO₂, Y₂O₃,SiO_(x)N_(y), Si₃N₄, HfO_(x)N_(y), AlO_(x)N_(y), TiO_(x)N_(y), MgF₂ oderCaF₂. Ion-enhanced evaporation or plasma-enhanced gas phase depositioncan be used as coating methods. Finally, several layers differing intheir composition and thickness can be applied consecutively, asdescribed in one of the embodiments portrayed.

[0065] Phase masks need not be produced directly by the two-beaminterference method but can be copied directly or indirectly from amaster copy thus produced. They can be used several times with layersinterrupted in diverse manner when diverse arrangements of couplinggratings and the like are to be generated while maintaining a constantgrating period. Instead of a nontransparent layer, a suitable layer oftransparent material can also be used. Thus, the grooves of the gratingcan be filled by a material having the refractive index of the phasemask substrate.

[0066] The phase mask can be antireflection-coated. In this case it maynot be necessary to apply a reflection-reducing layer to the photoresistlayer, which facilitates the production of series of grating structureson the substrates. For anti-reflection, a layer having a refractiveindex between that of the phase mask substrate and that of air, and forinstance consisting of MgF₂, is applied to the side of the phase maskfacing the photoresist layer. At the same time the grating must beadjusted in such a way that the interfering diffraction orders of thetransmitted light will again have the same intensity. This adjustmentcan occur by changing the groove-to-land ratio and the grating depth,which are readily calculated with the aid of programmes known amongexperts.

List of Reference Symbols

[0067]1 Substrate

[0068]2 Transparent layer

[0069]3 Coupling grating

[0070]4 Added top part

[0071]5 Cover plate

[0072]6 Opening

[0073]7 Tube section

[0074]8 Cavity

[0075]9 Optical system

[0076]10 Photoresist layer

[0077]11 Mercury-vapour lamp

[0078]12 Optical system

[0079]13, 13′ Folding mirror

[0080]14 Phase mask

[0081]15 Quartz substrate

[0082]16 Photoresist layer

[0083]17 Chromium layer

[0084]18 Photoresist layer

[0085]19 Glass plate

[0086]20 Layer

[0087]21 a,b Stripe waveguides

[0088]22 Coupling region

[0089]23 Coupler

[0090]24 Input

[0091]25 a,b,c Outputs

[0092]26 a,b Optical fibres

[0093]27 Photodetector

[0094]28 Laser

[0095]29 Optical fibre

[0096]30 Semi-transmissive mirror

[0097]31 First optical system

[0098]32 Second optical system

[0099]33 a,b Photodetectors

1. Process for producing at least one continuous grating structureformed as a line grating with distances of between 100 nm and 2500 nmbetween consecutive grating lines on a surface portion of a substrate,by covering the surface portion with a photoresist layer (10), bringingthe surface portion into the near field of a phase mask (14) having agrating structure, with the photoresist layer (10) facing this mask,exposing the phase mask (14) at an angle which departs from the Lithrowangle (Θ_(L)) or from 0° by no more than 10°, preferably by no more than5°, developing the photoresist layer (10) and subjecting the surfaceportion to an etch process to produce the grating structure, removingthe photoresist layer (10), characterised in that a substrate isstructured in advance by photolithography with the two-beam interferencemethod and the structure so produced or a structure derived from thesame is used as a phase mask (14) for the production of the at least onegrating structure and that the extension of the said structure is atleast 0.5 cm parallel to the lines.
 2. Process according to claim 1,characterised in that the extension of the at least one gratingstructure is at least 1 cm parallel to the lines.
 3. Process accordingto claim 1 or 2, characterised in that the surface area of the at leastone grating structure on the phase mask is at least 10 cm².
 4. Processaccording to one of the claims 1 to 3, characterised in that theexposure of the photoresist layer (10) is to a mercury-vapour lamp (11).5. Process according to one of the claims 1 to 3, characterised in thatthe exposure of the photoresist layer (10) is to an excimer laser orargon laser.
 6. Process according to one of the claims 1 to 5,characterised in that the phase mask (14) comprises a transparentsubstrate and a layer interrupted in a structured way opticallyinactivating the grating structure.
 7. Process according to one of theclaim 6, characterized in that the interrupted layer consists of anontransparent material, particularly metal, and preferably is achromium layer (7).
 8. Process according to claim 7, characterized inthat the substrate is a quartz substrate (15).
 9. Process according toone of the claims 1 to 8, characterised in that the side of the phasemask (14) facing the photoresist layer (10) is covered by anantireflection layer.
 10. Process according to one of the claims 1 to 9,characterised in that during the exposure of the photoresist layer (10)the latter is in vacuum contact with the phase mask (14).
 11. Processaccording to one of the claims 1 to 10, characterised in that thethickness of the photoresist layer (10) is at most 200 nm.
 12. Processaccording to one of the claims 1 to 11, characterised in that thephotoresist layer (10) prior to exposure is covered by areflection-reducing layer.
 13. Process according to one of the claims 1to 12, characterised in that during the exposure of the photoresistlayer (10), the distance between this layer and the phase mask (14) isbetween 2 μm and 100 μm.
 14. Process according to one of the claims 1 to13, characterised in that the etch process is reactive ion etching,preferably with a gas containing at least one of the followingcomponents: Ar, CHClF₂, CHF₃.
 15. Process according to one of the claims1 to 14, characterised in that the material of substrate (1) essentiallyis quartz, silicon, thermally oxidised silicon, germanium,silicon-germanium, a III-V compound semiconductor, or lithium niobate.16. Process according to one of the claims 1 to 15, characterised inthat at least one transparent layer (2) having a refractive indexdifferent from that of the substrate is applied to the surface portionafter applying the grating structure.
 17. Process according to claim 16,characterised in that the grating structure and the transparent layer(2) are formed in such a way that the coupling angle (Θ) changes by atmost 0.1°/cm along the line and the absolute value of deviation of thecoupling angle (Θ) from a desired value does not exceed 0.5°. 18.Process according to claim 16 or 17, characterized in that thetransparent layer (2) is applied by reactive DC magnetron sputtering, inparticular pulsed DC sputtering or AC-superimposed DC sputtering. 19.Process according to one of the claims 16 to 18, characterized in thatthe thickness of the transparent layer (2) is between 50 nm und 5000 nm.20. Process according to one of the claims 16 to 19, characterized inthat the material of the transparent layer (2) is Ta₂O₅, Nb₂O₅, TiO₂,ZrO₂, Al₂O₃, SiO₂—TiO₂, HfO₂, Y₂O₃, SiO_(x)N_(y), Si₃N₄, HfO_(x)N_(y),AlO_(x)N_(y), TiO_(x)N_(y), MgF₂ or CaF₂.
 21. Optical element, producedby the process according to one of the claims 1 to
 20. 22. Evanescentfield sensor plate with a platelike substrate (1) with, on a surfaceportion, at least one continuous coupling grating (3) formed as a linegrating with a grating period between 150 nm and 2000 nm which parallelto the lines extends over at least 0.5 cm and bears a transparent layer(2) with a refractive index different from that of the substrate (1),characterized in that the coupling angle (Θ) changes by at most 0.15°/cmalong the line and the absolute value of deviation of the coupling angle(Θ) from a desired value on the evanescent field sensor plate does notexceed 0.5°.
 23. Evanescent field sensor plate according to claim 22,characterized in that the extension of the coupling grating (3) alongthe line is at least 1 cm.
 24. Evanescent field sensor plate accordingto claim 22 or 23, characterized in that the surface area of thecoupling grating is at least 10 cm².
 25. Evanescent field sensor plateaccording to one of the claims 22 to 24, characterised in that thecoupling angle (Θ) changes by at most 0.05°/cm along the line. 26.Evanescent field sensor plate according to one of the claims 22 to 25,characterized in that the absolute value of deviation of the couplingangle (Θ) from its mean value on the evanescent field sensor plate doesnot exceed 0.3°, preferably not 0.15°.
 27. Evanescent field sensor plateaccording to one of the claims 22 to 26, characterised in that therefractive index of the transparent layer (2) is between 1.65 und 2.80.28. Evanescent field sensor plate according to one of the claims 22 to26, characterized in that the transparent layer (2) consists of Ta₂O₅,Nb₂O₅, TiO₂, ZrO₂, Al₂O₃, SiO₂—TiO₂, HfO₂, Y₂O₃, SiO_(x)N_(y), Si₃N₄,HfO_(x)N_(y), AlO_(x)N_(y), TiO_(x)N_(y), MgF₂ or CaF₂.
 29. Evanescentfield sensor plate according to one of the claims 22 to 28,characterized in that the thickness of the transparent layer (2) isbetween 50 nm and 200 nm.
 30. Evanescent field sensor plate according toone of the claims 22 to 29, characterised in that the groove-to-landratio of the at least one coupling grating (3) is between 0.3:1 and 3:1,preferably between 0.7:1 and 1.5:1.
 31. Evanescent field sensor plateaccording to one of the claims 22 to 30, characterised in that thegrating depth of the at least one coupling grating (3) is between 5 nmand 75 nm.
 32. Evanescent field sensor plate according to one of theclaims 22 to 31, characterised in that the at least one coupling grating(3) covers only part of the surface of the evanescent field sensor platewhile a remaining part remains free.
 33. Evanescent field sensor plateaccording to claim 32, characterised in that it has at least onecoupling grating (3) formed as a strip extending in parallel to thelines, essentially over the entire width or length of the evanescentfield sensor plate.
 34. Evanescent field sensor plate according to claim33, characterised in that several coupling gratings (3) in the form ofstrips are arranged at a distance parallel to each other.
 35. Microtitreplate with an evanescent field sensor plate according to one of theclaims 22 to 34 as well as with an added honeycomb-shaped top part (4)which laterally delimits each of the cavities (8) arranged in a regulararray, the bottom of each of the cavities being formed by the evanescentfield sensor plate.
 36. Optical coupler for communications technologywith a platelike substrate (1) with, on a surface portion, at least onecontinuous coupling grating (3) formed as a line grating with a gratingperiod between 100 nm and 2500 nm which parallel to the lines extendsover at least 0.5 cm and bears a transparent layer (2) with a refractiveindex different from that of the substrate (1), characterised in thatthe absolute value of deviation of the coupling angle (Θ) from a desiredvalue on the coupling grating (3) does not exceed 0.5°.
 37. Coupleraccording to claim 36, characterized in that the extension of thecoupling grating (3) along the line is at least 1 cm.
 38. Coupleraccording to claim 36 or 37, characterized in that the surface area ofthe coupling grating (3) is at least 10 cm².
 39. Coupler according toone of the claims 36 to 38, characterized in that the coupling angle (Θ)changes by at most 0.1°/cm along the line of the coupling grating (3).40. Coupler according to claim 39, characterized in that the couplingangle (Θ) changes by at most 0.05°/cm along the line.
 41. Coupleraccording to one of the claims 36 to 40, characterized in that theabsolute value of deviation of the coupling angle (Θ) from its meanvalue on the surface portion does not exceed 0.3°, and preferably doesnot exceed 0.15°.
 42. Coupler according to one of the claims 36 to 41,characterised in that the refractive index of the transparent layer (2)is between 1.65 und 2.80.
 43. Coupler according to one of the claims 36to 41, characterised in that the transparent layer (2) consists ofTa₂O₅, Nb₂O₅, TiO₂, ZrO₂, Al₂O₃, SiO₂—TiO₂, HfO₂, Y₂O₃, SiO_(x)N_(y),Si₃N₄, HfO_(x)N_(y), AlO_(x)N_(y), TiO_(x)N_(y), MgF₂ or CaF₂. 44.Coupler according to one of the claims 36 to 43, characterised in thatthe thickness of the transparent layer (2) is between 50 nm and 200 nm.45. Coupler according to one of the claims 36 to 44, characterised inthat the groove-to-land ratio of the at least one coupling grating isbetween 0.3:1 and 3:1, preferably between 0.7:1 and 1.5:1.
 46. Coupleraccording to one of the claims 36 to 45, characterised in that thegrating depth of the at least one coupling grating (3) is between 5 nmand 75 nm.
 47. Coupler according to one of the claims 36 to 46,characterised in that the surface portion bears at least two regularcoupling gratings (3 a, 3 b) with different grating periods.
 48. Coupleraccording to one of the claims 36 to 47, characterized in that thesurface portion bears at least one irregular coupling grating (3) inwhich the distance between neighboring grating lines is not constant.49. Coupler according to claim 48, characterised in that in theirregular coupling grating (3) the grating period only changes in adirection normal to the lines, preferably linearly.
 50. Device formonitoring a wavelength with a coupler (23) according to claim 48 aswell as with a detector assembly arranged directly beneath the couplerand having at least two photodetectors (33 a, 33 b) arrangedconsecutively normal to the lines.
 51. Device according to claim 50,characterised in that the detector arrangement can be displaced in adirection normal to the lines relative to the coupler (23).